Filter media for filtering liquids, also interchangeably referred to herein as liquid filtration filter media or LF filter media, are generally categorized in one of two types. A first type of filter media is comprised of conventional woven or nonwoven fibers. These LF filter media frequently have pore sizes greater than 1 micrometer (μm). A second type of LF filter media is porous film membranes, which are normally used in conjunction with a porous support layer. The porous film membranes usually have pore sizes less than 1 μm and are typically used to accomplish fine filtration of a liquid containing particulates. Examples of fine filtration are microfiltration, in which particulates having diameters in the range of from about 0.1 μm to about 10 μm are filtered from the liquid, ultrafiltration, in which particulates having diameters in the range of about 5 nanometers (nm) to about 500 nm are filtered from the liquid, and reverse osmosis, in which particulates having diameters in the range of about 1 angstrom (Å) to about 1 nm are filtered from the liquid. Sometimes the term “nanofiltration” is used for the filtration process to separate particles in the range of from 0.5 nm to 50 nm. Conventional LF filter media and porous film membranes are each suitable for use in microfiltration.
In order to try for particle size retention performance with nonwoven fiber-type LF filter media that is equivalent to performance obtained with porous film membranes having pore sizes of less than 10 μm, the nonwoven media have been made with increased depth, basis weight, or both. This results in increased tortuosity through which a particle would have to pass in order to escape capture by the medium, and increased particle holding capacity of the medium. Increasing the depth of fibers (e.g., number of fiber layers) in a nonwoven fiber-type LF filter medium, however, undesirably increases the pressure drop (also referred to as medium resistance) across the medium when in use. Increased pressure drop translates into increased energy for the filter medium user and shorter filter medium lifetime. Membranes offer a desirable improvement in fine particle retention but suffer increased pressure drop and reduced liquid flow rate (also referred to as flux).
Membranes also tend to be cost-prohibitive and do not provide a satisfactory contaminant-holding capacity over an entire range of pressure drop. These drawbacks limit the useful applications of porous film membrane-type LF filter media.
PCT International Patent Application Publication Number WO 2007/041311 mentions LF filter media having mean flow pore sizes of between about 0.5 μm and about 5.0 μm, inclusive. In WO 2007/041311, Table 1 mentions a lowest mean flow pore size of 0.55 μm for a filter medium of Example 8 having a basis weight of 88.1 grams per square meter (g/m2).
There is a need in the art for improved filter media.